1. Field of the Invention
The present invention relates to a sintered alumina ceramic and a method for producing the same, and to a cutting tool; in further detail, it relates to a sintered alumina ceramic having excellent strength and hardness at room temperature as well as at a temperature as high as 1000xc2x0 C. or higher, to a method for producing the same, and to a cutting tool having high-speed cutting ability and resistance against chipping.
2. Description of the Related Art
Since alumina ceramics are high in hardness but have low affinity with iron as compared with silicon nitride based ceramics and exhibit high chemical stability, alumina ceramics show excellent abrasion resistance. Thus, cutting tools made of alumina ceramics have been widely used heretofore for high-speed finishing of steel and cast iron. On the other hand, due to relatively low strength and toughness, alumina ceramics suffer from problems of chipping when they are used as cutting tools.
As a sintered alumina ceramic that has overcome the above problems, there is known, for instance, an alumina-TiC based material the strength and toughness of which are improved by dispersing TiC particles in the alumina sintered ceramic. In the case of such an alumina-TiC based material, the resistance against chipping when used as a cutting tool is considerably improved by adding TiC. However, since TiC is inferior to alumina in oxidation resistance and chemical stability with respect to iron, there still is a problem of lowered abrasion characteristics. Further, a method is proposed comprising adding MgO, i.e., a grain growth inhibitor, to pure alumina, such that fine texture and increased strength may be realized (e.g., xe2x80x9cPowder and Powder Metallurgyxe2x80x9d, 40(8), 805-808 (1993)).
Recently, however, further higher speed in cutting is required, and since the temperature of the blade front increases under such free cutting conditions, further improvement is required in high temperature hardness and high temperature strength. Since alumina-based tools proposed heretofore are insufficient in hardness and strength at high temperatures, they generate abrasion or flaking on high-speed cutting (at a cutting speed in a range of from 600 to 1000 m/min), and suffer problems of short tool life. Accordingly, there is a need to develop a sintered alumina ceramic having excellent strength and hardness at both room temperature and at high temperature, as well as an alumina-based tool having both high-speed cutting ability and resistance against chipping.
The present invention has been accomplished in the light of the aforementioned circumstances, and an object thereof is to provide a sintered alumina ceramic having excellent strength and hardness at room temperature as well as at a temperature as high as 1000xc2x0 C. or higher, a method for producing the same, and a cutting tool having high-speed cutting ability and resistance against chipping.
The present inventors have extensively studied the relation between the sintered alumina ceramics and their properties, and as a result, have found that, by controlling the particle size of the alumina particles and the content of SiO2 and other components contained in the sintered ceramic to a predetermined range, and by controlling the ratio of the peak intensity as measured by X-ray diffraction method of the specified compound of alumina and Group 3A metal oxide with respect to that of alumina contained in the sintered ceramic, both high strength and high hardness at room temperature and at high temperatures can be achieved; and that, when used in a cutting tool, can provide a cutting tool having both high-speed cutting ability and resistance against chipping.
The sintered alumina ceramic according to the present invention is characterized in that the alumina particles contained in the sintered ceramic have an average particle size of 4.0 xcexcm or smaller, the total content of alkali metal elements, alkaline earth metal elements, Si, and Ti as converted to oxides account for 0.1 mol or less per 100 mols of alumina contained in the sintered ceramic, and the total peak intensity for the principal peaks for ReAlO3 and Re3Al5O12 (Re: Group 3A metal of Periodic Table) with respect to the intensity of the principal peak of alumina as measured by X-ray diffraction method and calculated according to the following equation fall in a range of from 1 to 75%:
(A+B)xc3x97100/C 
where,
A: peak intensity of (112) diffraction peak of ReAlO3,
B: peak intensity of (420) diffraction peak of Re3Al5O12, and
C: peak intensity of (113) diffraction peak of alumina.
The cutting tool according to the present invention is characterized in that it is constructed of the sintered alumina ceramic of the present invention.
The method for producing a sintered alumina ceramic according to the present invention is a production method for the sintered alumina ceramic of the present invention above, and comprises preparing a raw material mixture of alumina and a Group 3A metal oxide or a Group 3A compound which provides a Group 3A metal oxide on heating, or a raw material mixture comprising a compound of alumina and a Group 3A metal oxide; producing a sintered ceramic having a relative density of 94.5 to 99.0% with respect to the theoretical density by firing the molding of the resulting raw material mixture; and subjecting the resulting product to hot isostatic pressing (which is referred to hereinafter as xe2x80x9cHIP treatmentxe2x80x9d) such that the relative density thereof becomes 99.0% or higher with respect to the theoretical density.
The sintered alumina ceramic according to the present invention possesses excellent strength and hardness at room temperature and at high temperatures of 1000xc2x0 C. or higher. Further, the method for producing a sintered alumina ceramic according to the present invention easily enables obtaining sintered alumina ceramics having the excellent characteristics above. Furthermore, since a sintered alumina ceramic having excellent strength and hardness at room temperature and at high temperatures of 1000xc2x0 C. or higher is employed, the cutting tool according to the present invention is equipped with both high-speed cutting ability and resistance against chipping, has long tool life, and can be favorably used for high-speed cutting.